The present invention relates to a blue-emitting phosphor for use in a fluorescent lamp and a fluorescent lamp employing the same, and particularly to an aluminate phosphor, as a blue-emitting phosphor which is mixed with a red-emitting phosphor and a green-emitting phosphor to be used in a three-band type fluorescent lamp with a high color-rendering properties, which has a uniform particle size distribution and improved suitability to a phosphor layer, and to a fluorescent lamp which employs the aluminate phosphor to increase in luminous flux intensity.
Generally, a fluorescent lamp for illumination uses a phosphor which absorbs the 254 nm Hg-resonance wave and is activated so as to convert the ultraviolet luminescence of mercury vapor into visible light. In a conventional fluorescent lamp for illumination, a white-emitting calcium halophosphate phosphor, such as Ca.sub.10 (PO.sub.4).sub.6 (F,Cl).sub.2 :Sb,Mn, has usually been used. Recently, in order to improve the color-rendering properties and emission output of fluorescent lamps, a three-band type fluorescent lamp which employs the proper mixture of red, green and blue-emitting phosphors whose emission spectrum occupies a relatively narrow band, has been put to practical use. For instance, for the blue-emitting phosphor, europium-activated barium magnesium aluminate phosphor (BaMg.sub.2 Al.sub.16 O.sub.27 : Eu.sup.2+), for the green-emitting phosphor, cerium and terbium-activated magnesium aluminate phosphor [(Ce, Tb)MgAl.sub.11 O.sub.19 ], and for the red-emitting phosphor, europium-activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu.sup.3+) are mixed in adequate ratio.
In such a three-band type phosphor lamp, since the emitting colors of the respective phosphors are considerably different from one another, if the emitting intensity of a corresponding phosphor is decreased when used in the fluorescent lamp, color deviation occurs, degrading the color-rendering properties.
As the blue-emitting phosphor used in such a three-band fluorescent lamp, BaMgAl.sub.16 O.sub.27 :Eu.sup.2+ is disclosed in Japanese patent publication sho 52-22836, in which the emitting intensity is undesirably reduced with the elapse of time. Due to this, when the blue-emitting phosphor is used in the three-band type fluorescent lamp, since divalent europium-activated barium magnesium aluminate phosphors suffer more sever deterioration than the green-emitting phosphor (for instance, cerium and terbium-activated magnesium aluminate phosphor) and the red-emitting phosphor (for instance, europium-activated yttrium oxide phosphor) which are used together with the blue-emitting phosphor, in the three-band type fluorescent lamp using the divalent europium-activated barium magnesium aluminate phosphor as the blue-emitting phosphor, the emitting color varies greatly during the use of the lamp. Therefore, a blue-emitting phosphor for use in a fluorescent lamp, having a high emission intensity and at the same time a low reduction rate of the emission intensity during the use of the lamp, is required.
U.S. Pat. No. 4,249,108 discloses blue-emitting Eu-activated phosphor compositions within a La.sub.2 O.sub.3 --MgO--Al.sub.2 O.sub.3 system. U.S. Pat. No. 4,733,126 discloses a blue-emitting aluminate phosphor expressed as a general formula of (Ba.sub.w Ca.sub.x Mg.sub.y Eu.sub.z)O.aAl.sub.2 O.sub.3.bSiO.sub.2 where 0.1.ltoreq.w.ltoreq.0.5, 0.0004.ltoreq.x.ltoreq.0.0020, 0.1.ltoreq.y.ltoreq.0.8, 0.03.ltoreq.z.ltoreq.0.10, w+x+z=1, 1.5.ltoreq.a.ltoreq.4.5, and 0.0005&lt;b.ltoreq.0.0030.
Further, Japanese laid-open patent publication No. sho 61-254689 discloses a phosphor expressed as a general formula of (Ba,Eu)O.xMgO.yGd.sub.2 O.sub.3.zAl.sub.2 O.sub.3 where 0.8.ltoreq.x.ltoreq.3.0, 0&lt;y.ltoreq.0.05, and 4.ltoreq.z.ltoreq.8. Here, a small amount of gadolinium oxide is added to the barium magnesium aluminate phosphor activated by divalent Eu to stabilize the phosphor crystal so that the decrease of the emission intensity of the phosphor is prevented and the deterioration of the fluorescent lamp is minimized while in use.
However, such aluminate phosphors are disadvantageous in that they have poor suitably to a phosphor layer because the particles of the phosphors themselves are difficult to control.